Overview: XML, HTML, and all that

Jon Bosak
Sun Microsystems
April 11, 1997

For best display, set font size to 24pt.

What is text, really?

Structured markup: the basic idea

Why structured markup?

The separation of presentation from structure and content makes possible

multiple targets (both mechanical and human)
reusable information
intelligent downstream document processing
large-scale information management

Is HTML structured markup?

No, not really
HTML maximizes ease of use by providing one general hardwired tag set for all applications and predefining certain behaviors for each tag
HTML is perfectly suited to the applications for which it was designed, but it was not designed to handle all applications
In particular, HTML was not designed to handle very large or complex collections of data that need to be used in different ways or maintained over a long period of time

Specific features missing from HTML

HTML does not provide a way to validate document data structures or impose editorial control in projects with multiple authors
It is difficult or impossible in the general case to reliably generate navigational aids from HTML documents
As a result, navigation usually has to be implemented by adding handcrafted hypertext links
HTML browsers have no concept of entity management (no mechanism for modular reuse)
With just a general-purpose tag set, context searching or other behavior based on semantic distinctions is difficult or impossible
HTML cannot easily be made to model data structures residing in databases

HTML is not the optimum data format for database interchange or certain kinds of large-scale commercial publishing.

What is SGML?

Standard Generalized Markup Language: the international standard for structured document interchange, ISO 8879 (1986)
Developed out of a search for a universal typesetting language begun in the late 1960s
Descriptive, not procedural -- the procedures are left to formatting/presentation systems
Not a single markup language, but a metalanguage for the specification of an unlimited number of markup languages, each optimized for a particular category of documents
The SGML description of a markup language is called a Document Type Definition (DTD)
In SGML, the DTD is a required part of the document

Major industry DTDs (markup languages)

ATA 2100	aircraft industry
CALS	military, aerospace
CMC	pharmaceuticals
PCIS	semiconductors
DocBook	computer software
IBMIDDoc	IBM software
SAE J2008	automobile manufacturing
TMC T2008	truck manufacturing
TIM	telecommunications
EDGAR	Securities and Exchange Commission
ISO 12083	journal, book, and magazine publishing
ICADD	publishing for the print-disabled
TEI	academic and scholarly publishing
UTF	news media
HTML	World Wide Web

Advantages of generic SGML

Based on international standards -- no vendor wars
Fully extensible -- no tag limitations
Supports formal validation and controlled authoring
Highly structured -- can model any kind of data
Links and navigational aids can be generated directly from the structure of documents
Context searching increases the speed of user access 10-20 times
Documents can easily be reused for different purposes
All versions of a document (printed and online) can be generated from the same source
User-selectable stylesheets allow dynamically configurable views
System administration of large document repositories is vastly simplified

Why not just put SGML on the Web?

SGML does provide the key features needed to support future large-scale data-intensive Web applications...

Extensibility
Structure
Validation
Late binding of presentation

...but SGML is too big and far too complex for most Web software developers (not to mention site administrators).

The attempt to put SGML on the Web

W3C activity "Generic SGML on the Web" suggested at WWW5 in Paris and initial participants recruited at SGML Europe in Munich (May, 1996)

"The goal of the W3C SGML activity is to enable generic SGML to be served, received, and processed on the Web. As in the case of HTML, the implementation of SGML on the Web will require attention not just to structure and content, but also to the standardization of linking and display functions."

Syntax
Linking
Styles
Chunking (outsourced to SGML Open)

What actually happened

July-August 1996: assembled 60 of the world's top SGML experts to form W3C SGML WG; formed 11-member W3C SGML ERB to make decisions and steer the activity
September-November 1996: threw out about 80 percent of the SGML standard
Result: Part 1 of the XML specification (the "red book")

XML Part 1 is a self-contained easy-to-implement subset of SGML for use on the Web.

Current working draft of Part 1: http://www.w3.org/pub/WWW/TR/WD-xml-lang-970331.html

XML syntax vs. HTML syntax

Freely extensible
Arbitrarily deep structure
Optional validation
Strong separation of content and presentation

XML vs. SGML

Vastly simpler (a very small conforming subset of SGML)
Vastly easier to implement
Design is formal and concise ("Lexable and Yaccable")
Retains most of SGML's power as a document delivery medium
DTD is optional
Can (and will) function as an authoring format, but functions best as a delivery format from databases (primary design goal)

Current overview of the activity

The XML activity can be summed up as the adaptation of existing international publishing standards for use on the Web.

Phase I: Subset SGML (ISO 8879) for easy implementation
Phase II: Design standard hypertext mechanisms based on HyTime (ISO/IEC 10744)
Phase III: Design a standard stylesheet for arbitrarily structured information by subsetting DSSSL (ISO/IEC 10179)

Is XML intended to replace HTML?

In a word: No.

HTML is well-suited to a wide variety of current applications. It isn't broken!
XML provides a more complex set of tools for addressing a different set of applications:
- Database exchange
- Distribution of processing to clients
- Client-side manipulation of views into the data
- Customization of information by intelligent agents
- Management of document collections

Major XML application area #1: Database interchange

Example: Health care data

Bogus Web solution

Real Web solution

Why HTML can't handle the health care example

The HTML tag set is too limited to represent or differentiate between the multitude of database fields in the mixture of documents making up the patient's medical history.
HTML is incapable of representing the variety of structures in those documents.
HTML lacks any mechanism for checking the data for structural validity before the receiving application attempts to import it into the target database.

Generalized data exchange

The role of a hub format

Other applications fitting the database interchange model

Legal publishing
The government drug approval process
Collaborative CAD/CAM efforts
Collaborative calendar management across different systems
Any corporate network application that works across databases, especially where policies must be enforced: purchase orders, expense requests, etc.
Exchange of information between players in any broker-organized business: insurance, securities, banking, etc.

Major XML application area #2: Distributed processing

Example: semiconductor data

Why HTML can't handle the semiconductor example

It requires industry-specific markup that cannot be implemented within the confines of the fixed HTML tag set.
It requires that the data representation be platform- and vendor-independent so that data from a variety of sources can be used to drive a variety of distributed applications.

This is a widely applicable model!

Other applications fitting the distributed processing model

Design applications where the a designer considers various alternatives: electronics, engineering, architecture, menu planning, etc.
Scheduling applications where a customer explores various possibilities: airlines, trains, buses, and subways; restaurants, movies, plays, and concerts.
Commercial applications that allow consumers to explore alternatives: real estate, automobiles, appliances, etc.
The entire spectrum of educational applications.
The entire spectrum of customer-support applications, including online help.

Major XML application area #3: user views of the data

Example: Views of our Solaris documentation, server-mediated...

... and client-mediated

Other applications that need user-controlled views

An installation sheet that carries warnings in multiple languages can be made to show just the ones in the language selected by the user.
A document containing many annotations can be switched from a mode that shows only the text, to a mode that shows only the annotations, to a mode that shows both, just by making a menu selection.
A phone book sorted by last name can instantly be changed into a phone book sorted by first name.

Major XML application area #4: Web agents

Example: the 500-channel TV guide

Once again, a category of applications depends on the ability to standardize on a form of data representation for a particular industry or problem domain.

Hyperlinking in XML

(Tim Bray will present this.)

XML semantics

How do we make XML documents do something?

Stylesheet-based approaches
- CSS
- DSSSL
Programmatic approaches
- Java
- Other

CSS (Cascading Style Sheets)

Easy syntax
Good match for HTML
Will be widely used for simple XML documents
Embedded CSS style attributes will be a common feature in generated XML

Come to this afternoon's XML technology demo session to see what can be done with CSS and XML.

Limitations of CSS in complex applications

CSS cannot grab an item (such as a chapter title) from one place and use it again in another place (such as a page header).
CSS has no concept of sibling relationships. For example, it is impossible to write a CSS stylesheet that will render every other paragraph in bold.
CSS is not a programming language; it does not support decision structures and cannot be extended by the stylesheet designer.
CSS cannot calculate quantities or store variables. This means, at the very least, that it cannot store commonly used parameters in one location that is easy to update.
CSS cannot generate text (page numbers, etc.)
CSS uses a simple box-oriented formatting model that works for current Web browsers but will not extend to more advanced applications of the markup, such as multiple column sets.
CSS is oriented toward Western languages and assumes a horizontal writing direction.

Document Style Semantics and Specification Language (DSSSL)

The International Standard for specifying the formatting of structured documents (ISO/IEC 10179:1996)
Two distinct independent languages:
- Transformation language
- Style language
Each of which uses two other languages defined by the standard:
- The DSSSL expression language
- The Standard Document Query Language (SDQL)
DSSSL syntax is derived from Scheme
- Functional (side-effect free)
- Turing complete

Some DSSSL stylesheet snippets

(element emph
  (make sequence
    font-weight: 'bold))

(define monospace-font-family "Courier")

(element code
  (make sequence
    font-family: monospace-font-family))

(declare-initial-value writing-mode 'left-to-right)
(declare-initial-value font-size 12pt)
(declare-initial-value line-spacing 14pt)
(declare-initial-value font-family "Helvetica")

(define paragraph-indent 24pt)

(element p
  (make paragraph
    first-line-start-indent: (if (first-sibling?)
                                 0pt
                                 paragraph-indent)))

(define chapter-title-style
  (style
    font-size: 18pt
    line-spacing: 24pt
    quadding: 'center))

(element (chapter title)
  (sosofo-append
    (make paragraph
      use: chapter-title-style
      span: 2
      (literal "Chapter "
               (format-number (ancestor-child-number) "1")))
    (make paragraph
      use: chapter-title-style
      span: 2)))

(define (expt b n)
  (if (= n 0)
      1
      (* b (expt b (- n 1)))))

DSSSL: Mixed languages

DSSSL: Mixed scripts

DSSSL: Top-to-bottom languages

DSSSL: Margin attachments

DSSSL: Rotated text areas

DSSSL: Multiple columns

DSSSL: Variant column flows

DSSSL: Column spans and zones

DSSSL: Synchronized columns

DSSSL: Asian language features

glyph annotation
multi-line inline notes
emphasizing marks

DSSSL: Advanced mathematics formatting

mark
fence
fraction
radical
grid

DSSSL demonstration: Jade

   http://www.jclark.com/jade

XML/Java demonstration: Jumbo

   http://www.venus.co.uk/
	     ~pmr/demos.tar.gz
   http://www.venus.co.uk/
	     ~pmr/README

Alternative source:

   http://www.nottingham.ac.uk/
	     ~pazpmr/demos.tar.gz
   http://www.nottingham.ac.uk/
	     ~pazpmr/README